Descriptions

A computer model was created using compressor and
condenser performance data to simulate vapor-compression
ammonia refrigeration system performance under various
operating conditions and strategies. The model was used to
estimate energy, demand, and cost savings resulting from
reduced condensing pressures.
Two ammonia refrigeration systems were selected for the
investigation into reduced condensing pressure operation.
Each refrigeration system was monitored under two modes of
operation: high (fixed-head) and reduced (floating-head)
condensing pressure. Compressor and condenser fan power,
condensing pressure, and ambient dry and wet-bulb
temperatures were recorded for a one week period. During
the fixed-head period, the condensing pressure was
maintained at 145 to 155 psig, which is typical for
industry. The condenser fan controls were adjusted for the
floating-head period, reducing the average condensing
pressures to 117 to 130 psig. Reductions of 9.8% to 11.7%
in combined compressor and condenser fan power were observed
during the floating-head operation. In addition, none of
the potential problems associated with floating-head
operation were observed.
The model was used to simulate refrigeration system
performance during the monitoring periods, using weather
data and estimated refrigeration loads as model input.
Model predictions were compared with the monitored data to
validate the model performance.
The model was used to predict monthly and annual
energy, peak-demand, and operating costs for a range of
condensing pressures. Monthly dry and wet-bulb temperature
profiles, and estimates of refrigeration loads were used as
input for the model.
The model predicted:
Annual energy savings of 10.6% to 12.0%,
corresponding to 0.63% to 0.83% savings per degree
reduction in average condensing temperature.
Average monthly peak demand savings of 8.7% to 9.2%,
corresponding to 0.53% to 0.64% savings per degree
reduction in average condensing temperature.
Average annual cost savings of 10.4% to 11.9%,
corresponding to 0.62% to 0.82% savings per degree
reduction in average condensing temperature.
The possibility of minimum condensing pressures,
below which the condenser fan operating costs
outweigh compressor savings.
A significant portion of the possible energy savings
(79% to 93%) with no additional equipment or capital
cost.